I was looking for a way to manage several inputs and several outputs for all my devices around the perimeter of my apartment so I don't hear neighbors and they don't hear me, and only secondarily, so I can hear one audio source in every room. I considered buying another mixer or getting my antique Tangent mixer out of storage, but they don't have enough outputs. I decided to build my own but after searching ebay, it seems most reasonable to start with completed or partial circuits and put them together like Lego bricks, and add some circuitry around them.

Please critique the following plan:

I already have a collection of quality old school speakers, but I'm sure I can find more in the used market, especially thrift shops. I saw a huge 4ohm subwoofer for cheap, but had no way to drive it... I could have bought it and then found the right amp for around $10.

I just purchased several complete pre-amp boards with a dual op-amp socket each (under $2), and after I get those working, may add at least one bluetooth receiver (under $4), and any other preamp board can be added..

I'm going to just use each preamp for stereo, but for a pro-audio build, use each for balanced line.

Purchase several complete amplifier boards under $10, one for each pair of speakers (or sub) throughout my home.
I don't think they need to match from room to room.

The cables that go from room to room could just be speaker wire running a higher current line level than consumer, in fact, the same potential as inside my old Tangent mixer (70's electronics are very easy to read). Could use balanced line from room to room to eliminate interference, but doubt there's an audible difference.

I'll need to add circuitry to gang the pre-amp outputs, correct offsets (not strictly necessary), add noise gates(?), and add volume controls for each and the resulting mix. Most obviously, just use a pot on each output and gang them together and then run them through another pot, no balance or tone. It seems like I may need another amp stage there, but I'm not sure. I won't bother with balanced line, unless there's no other solution to a potential ground loop, but would if I had a big house.

Instead of put everything in one box, I want to put each amp in the room with its pair of speakers, with a local circuit for off/main/local line or bluetooth and volume or at least mute.
Should I do anything to isolate the pre-amp outputs?
Do I handle volume between pre-amps and amps as just explained, or start over and look at active control?
If an amp is powered by a different circuit in the home than the preamps, how do I avoid ground loop issues?

This seems more scalable than practicality requires. Eight stereo inputs and eight stereo outputs seems sufficient for a home, though doubt I need more than half of that capacity. For pro audio, use each preamp for a balanced line instead of stereo.

I know this <can> be done with cheap little Chinese boards, but can it be done well?

If this works out nicely, I'll write a build thread. Your help is appreciated!

Ouch, not one reply. Okay.
Please tell me if it was the plan, some huge error, of this simply isn't the right place for it?
FYI: I was on the forefront of digital mixing as it was invented in the 90's, and have likely spent $10K on gear. I started my search by looking for circuit diagrams for single op amp preamps and really low power high end sound. It does not appear that I should build it, but at least start with the tiny rebuildable boards and redesign them if necessary. I had a TA2024 in ~2004 when they were demo units, which really drives towers nicely - at only 7W. Finding that I can buy a copy, and that there is a wide range to choose from means I don't need to build the pre-amps nor the amps, but am not sure if I should add protection circuits and signal matching. 7W of audiophile sound in eight locations in 600' is more than sufficient, for up to eight inputs all located in one location. Since there's distance between the preamps and amps in my 'hypothesis' then I may as well give each location the ability to play something local. That isn't quite pro-audio, but frankly if I had an upcoming gig, I'd rent equipment. This is just home use, based on my previous experience. Seems crazy to have more control than a live theatre, but why not?

It is not clear to me what you want to do. You have speakers, and want to.... ? Put a $10 7 Watt amp in each room? Get signal from another room or locally? Could you possibly want 8 different sources to 8 different rooms at once?

Those low-price "Chinese amps" come in every shade from quite-fine to crap, but are cheap enough to just try a few.

I would throw an unbalanced line from recordplayer to bedroom and see if it hummed. Expand as needed. There is clearly no One Way to do whatever it is you want to do.

There are RCA-Cat5 adaptors which would greatly cut wiring costs. On a casual listen they were blameless, but I am no gold-ear.

Thanks for replying. My apartment building has no privacy, and a lot of electrical interference. All my source equipment is in one room, and a set of wires can go to each pair of speakers in every corner of the apartment. I guess I do need to add an amp stage after the preamps, for enough current to drive 4-8 small amps, to give roughly a strong line level, so just audio cables should suffice. Do you think RJ45 would be better for to avoid interference? I could switch to balanced line since the socketed preamp stages are cheap, even if I use smaller cables.

All the equipment is in one room, so all the preamps would be in one circuit with the gain controls. Line level stereo then gets run to each room, which I was thinking of standard speaker wires. Since each room's amp is independent, I may as well give it a mute and the ability to use a local source.

Chinese boards will take trial and error, but some intentionally use obvious designs and fat traces so the components can be upgraded. My original TA2024 was a manufacturer's demo, and it really thumped giant speakers at only 7W per channel at audiophile quality (15W max); and there appear to be many similar designs now.

Did I say $50? That would be a basic implementation. I can budget $200 on this without speakers.

Do you have a method of mixing digital inputs and sending them to digital several digital outputs, without a computer?
What I described is like tearing apart a 8x8 live board and putting the pieces in different rooms. I used a live board in my last house, but don't have room nor need of it here, and it doesn't have enough outputs.

Optical really is the way to go if you have a lot of interference. (or network based)

Edit:
Or you could use network distributed audio (VBAN), it has much higher potential for sound quality than Bluetooth, really low latency (depends on your local hardware), but requires at least one computer for distribution, and compatible units for decoding, this can be used through wi-fi. Cheapest solution is probably Voicemeeter Banana, you can also use Dante Via, but they have a more expensive licensing system.

KaffiMan: I like that idea, so I just tried to design a system that way. When you add the known parts up, they cost more than this analog project, and I found no way to mix a bunch of Toslink signals together or even handle volume per signal without yet another device or a full computer. It seems untenable to convert everything then convert it back.

PRR: Several sources are in one room, to be mixed to stereo, going out to the other rooms equally, with volume to each room central or in that room, with the rooms having the ability to at least mute and switch to another source like a phone. If RJ45 will carry the signal adequately, wouldn't heavy exterior phone wires (two twisted pairs) carry stereo to each room using whatever connectors I have on hand?

VBAN: It does not have to be RJ45, easiest would be wi-fi access point and a small cheap tablet or phone supporting wi-fi (often such devices get discarded when the batteries are tired and can be powered through external psu to the right pins), both Apple and Android products are compatible with the VBAN Receptor. read up here:VB-Audio Network

You just need one computer for hosting. But the major benefits are: there's not much cabling required, noise is no concern (depends on the quality of each output device). And the volume can be controlled both centrally through the host computers volume control (external volume dials are an option), and on each device.

The reason noise is no concern is that: Audio is not transmitted through the cables, just a digital signal like regular network traffic. It's not audio until it's interpreted by the VBAN receptor.

Edit:
How where you going to solve it through Bluetooth anyway?
Reduction of signal quality ignored: How are you going to connect 8 bluetooth devices to one single transmitter?

I've got to say, VBAN looks very cool, but I'd have to have a computer at each location plus conversion to and from that system. I'd only need one computer for hosting? But it looks like each location needs a 'VBAN receptor' program operating, suggesting each location has to have a computer. That's a great solution, but isn't it overkill?

Because of neighboring signals, wifi and Bluetooth don't work farther than a room. I'd use Bluetooth in the other rooms as an alternate local source at $4 per room.

I could just build this all in one box and string speaker cables across the apartment, but that doesn't really simplify the design.
Preamps and volume controls in one room, high current capacity line level from room to room, amps mounted on or with speakers. What am I missing?